The function wp_get_active_network_plugins() is private to WordPress core. It is defined only when Multisite loads core files such as wp-includes/ms-load.php. When a theme, plugin, or custom script calls the function outside of that context, PHP throws a fatal error.
Typical triggers include using the function on a single‑site install, loading it before Multisite bootstraps, or having corrupted core files that never include the definition.